BACKGROUND Exertional heat illness (EHI) is recognised in horses, but few reports have investigated its risk factors. OBJECTIVES To identify risk factors for EHI in racehorses participating in flat races in Japan. STUDY DESIGN Descriptive epidemiology and retrospective unmatched case-control study. METHODS Between 2005 and 2016, veterinary records of horses diagnosed with EHI after flat races were reviewed retrospectively and data of the months from April to September were used for a case-control study. For each case, three control horses were randomly selected from starts between April and September. Race records of horses and estimated wet-bulb globe temperature (WBGT) indexes at the local meteorological observatory closest to the racecourse were investigated. To identify risk factors for EHI, univariable and multivariable logistic regression analysis was used. RESULTS Of 194 cases during the study period, 188 cases occurred between April and September. The highest incidence risk was in July (1.1 cases per 1000 starts, 95% confidence interval 0.84-1.45). In the final multivariable model, WBGT index, sex, race distance, age and bodyweight were associated with EHI. When WBGT index exceeded 28°C, the risk of EHI was considerably higher than <20°C (OR 28.5, 14.2-62.4, P<0.001). Compared with uncastrated males, geldings (OR 4.9, 1.8-13.3, p = 0.002) and females (OR 2.4, 1.5-3.7, P<0.001) were at high risk of EHI (P<0.01). Furthermore, races of >1600 m (OR 1.8, 1.2-2.8, P = 0.002), 4-year-old (OR 3.5, 1.6-7.9, P = 0.002) and ≥5-year-old (OR 3.9, 1.8-9.2, P = 0.001) horses and horses with low bodyweight (OR per 20 kg, 0.8, 0.7-1.0, P = 0.02) were associated with increased risk of EHI. MAIN LIMITATIONS The median straight-line distance between the racecourse and the local meteorological observatory was 14.2 km (range, 1.1-28.3 km). There was a lack of objective criteria of EHI due to the retrospective nature of the study. CONCLUSIONS We identified specific risk factors for EHI in racehorses. These results may be useful to the equine industry for reducing EHI occurrence in racehorses.

中文翻译：

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日本（2005-2016 年）纯种赛马在平地比赛中劳累性热病的危险因素。

背景 劳累性热病 (EHI) 在马中得到认可，但很少有报告调查其危险因素。目的 确定日本参加平地比赛的赛马 EHI 的危险因素。研究设计 描述性流行病学和回顾性无与伦比的病例对照研究。方法 2005年至2016年，回顾性审查在平地赛马后诊断为EHI的马的兽医记录，并将4月至9月的数据用于病例对照研究。对于每种情况，从 4 月至 9 月开始随机选择三匹对照马。调查了离赛马场最近的当地气象台的马匹比赛记录和估计的湿球温度（WBGT）指数。为了识别 EHI 的风险因素，采用单变量和多变量逻辑回归分析。结果研究期间194例病例中，4-9月发生188例。发病风险最高的是 7 月（每 1000 例发病 1.1 例，95% 置信区间 0.84-1.45）。在最终的多变量模型中，WBGT 指数、性别、种族距离、年龄和体重与 EHI 相关。当 WBGT 指数超过 28°C 时，EHI 的风险显着高于 <20°C (OR 28.5, 14.2-62.4, P<0.001)。与未阉割的雄性相比，阉马（OR 4.9, 1.8-13.3, p = 0.002）和雌性（OR 2.4, 1.5-3.7, P<0.001）的EHI风险较高（P<0.01）。此外，>1600 m (OR 1.8, 1.2-2.8, P = 0.002)、4 岁 (OR 3.5, 1.6-7.9, P = 0.002) 和 5 岁以上 (OR 3.9, 1.8- 9.2, P = 0.001) 马和低体重马 (OR 每 20 公斤, 0.8, 0. 7-1.0，P = 0.02）与 EHI 风险增加相关。主要限制 赛马场与当地气象台之间的中位直线距离为 14.2 公里（范围，1.1-28.3 公里）。由于该研究的回顾性，缺乏 EHI 的客观标准。结论 我们确定了赛马 EHI 的特定风险因素。这些结果可能有助于马业减少赛马中 EHI 的发生。